This proposal is an extension of R37 AI027998, a MERIT award to Marc Jenkins to study the role of D28 family proteins in the provision of costimulatory signals to CD4+ T cells. Significant progress was made in the first 4 years of funding, resulting in 6 senior author paper and 2 middle author papers by Dr. Jenkins. Two of the senior author papers were published in Immunity, 2 in PNAS and 1 in Nature Immunology. These papers and one about to be submitted describe achievement of may ofthe goals of original Specific Aim 1 including development of a sensitive peptide:major histocompatibility complex II (p:MHCIl) tetramer-based cell enrichment assay to detect naive polyclonal p:MHCII-specific CD4+ T cells, validation ofthis assay as a means to monitor clonal expansion, contraction, and memory cell formation from naive p:MHCII-specific precursors during a physiologically relevant bacterial infection, use ofthis assay to show that the major function of CD28 in vivo is to sustain clonal expansion through a signaling pathway involving NFkB but independent of the 2 suspected motifs in the CD28 cytoplamic tail, and identification of the role of CD28 family protein Inducible Costimulator (ICOS) in the formation of central memory cells and follicular helper cells. Progress was also make in Specific Aim 2 including validation of p:MHCIl tetramer-based cell enrichment as a means to study tolerance within polyclonal self-reactive T cell populations in normal repertoires and identification of candidate self antigens of potential relevance to autoimmunity that develops in mice lacking the inhibitory CD28 family protein CTLA-4. Aims forthe extension period include determination of whether CD28 signaling enhances TCR recruitment to the immune synapse in vivo and identification of MHCII-binding peptides in autoantigens from CTLA-4-deficient mice, production of p:MHCII tetramers containing these peptides, and use of these tetramers to determine if CTLA-4 controls autoimmunity by killing self pMHCII-specific T cells, making them anergic, or converting them into regulatory T cells.